The present invention relates generally to breakaway devices for rigid poles and posts implanted along roadways. The purpose of a breakaway device is to limit the amount of force and energy imparted to an impacting vehicle and its occupants by incorporating some form of failure mechanism into the pole base or into the pole itself.
With the ever increasing presence of the automobile on roadways and the corresponding increase in the number of utility poles, luminaires and road signs ground-mounted at the side of roadways, it became evident years ago that the risk of vehicles crashing into these ground-mounted structures would also become increasingly greater.
In a report entitled "Accident Analysis-Breakaway and Non-Breakaway Poles Including Sign and Light Standards Along Highways" Chapter 2, by King K. Mac and Robert L. Mason, DOT-HS-805-605, August, 1980, the authors reported that single-vehicle fixed object accidents account for only a small portion of all reported accidents. However, there is a disproportionate number of fatal accidents resulting from these relatively few accidents. Poles, such as utility poles, luminaires, sign or traffic signal supports are the most frequently struck objects in urban areas and among the more frequently struck objects in rural areas. The resultant injury severity of pole collisions, particularly those involving utility poles, is very high. Over half of these pole impacts are injury producing, and the injury frequency ranks right behind that corresponding to rollovers and impacts with bridge embankments.
In an effort to lessen the effects of these vehicular/pole collisions, D. L. Hawkins of the Texas Highway Department conceptualized the idea for slip base mounts for ground-mounted signs in the mid-1960's. The basic idea behind the slip base is to prevent a ground-mounted structure from rigidly resisting the impact momentum of an impacting vehicle by slipping off of its base in response to the force of the impact.
Breakaway designs for these ground-mounted structures were first applied for luminaires. Basic designs were classified in four categories namely (1) frangible base, (2) progressive shear, (3) slip base, (4) other types of breakaway designs.
With any type of breakaway system, the design criteria were to decrease as much as possible the momentum change to the vehicle from the time the vehicle impacts the ground-mounted structure until it passes through the structure after breakaway. According to present safety standards, an impacting vehicle should experience no more than a 1,000 lb.-sec. momentum change and preferably less than 750 lb.-sec. If the momentum change is kept below the maximum values, the likelihood of occupants surviving a pole impact is much greater.
The frangible and progressive shear bases used in connection with luminaires are designed to either fracture or shear metal in the base at a specified base fracture energy. These designs, however, are only good for aluminum or other metal type mounts where the frangible characteristics can be built into the pole and mount.
The slip base design utilized in connection with luminaries uses a totally different concept. It consists generally of two plates with cut slots. One plate is welded to the base of the luminaire pole and the other plate is welded to a luminaire mount which is embedded into the ground. Bolts are inserted into the slots to keep the two plates together. The bolts are tightened to a predetermined torque. Upon impact, the top plate, with the luminaire support attached, tends to slide in the direction of vehicle travel. Thus, the bolts are forced to slide out of the slots, freeing the luminaire support to move with the vehicle.
Similar slip base designs have been utilized in connection with sign supports. Often the slip base will be inclined at an angle such that an impacting vehicle will give an upward acceleration to the support and sign so that the sign will pass over the top of the vehicle.
The Texas Transportation Institute has developed a notable slip base design utilizing a triangular shape with slots situated at the corners of the triangle. It is designed such that it is capable of breaking away or slipping off the base when a vehicle impacts at sufficient speed from any direction.
Although these types of slip designs have been used in connection with luminaires and sign or signal supports, there has been no indication that the same types of slip designs could be utilized in connection with timber utility poles. It has been previously thought that such timber poles were too heavy for treatment using conventional slip base technology.
In fact, up until approximately 1982, most of the work to apply breakaway technology to timber utility poles involved various arrangements of holes, grooves and saw cuts placed at strategic locations in the pole base used to weaken the pole so that it would fall or fail more easily during a vehicle impact. However, this produced problems relating to the downing of the conducting wires and transformers which created significant safety hazards, significant power outages and significant repair costs. Thereafter, another weakened zone was introduced near the top of the pole underneath the conducting wires so that the entire middle section of the pole would breakaway leaving the top portion still connected to the conductors. These designs proved to be undesirable due to the total unpredictability of the time and mode of failure. Specifically, the bored holes, saw cuts or grooves severely reduced the pole's resistance to environmental loads creating a risk that the pole would break under severe ice and/or winter storm conditions which could then create a disastrous chain reaction of downed poles. The boring of holes in the pole would also greatly decrease its service life, rendering it more susceptible to rotting. See Mac and Mason supra.
Various slip base designs were studied and tested for use in connection with utility poles. A design which has met with some approval is an adaptation of the triangular, three bolt multi-directional slip base referred to above and used in connection with luminaire and sign breakaway supports. Another design is the Hawkins Breakaway System (HBS), designed by the inventor of the subject invention and reported in a government report entitled "Safer Timber Utility Poles" by Don L. Ivey and James R. Morgan, DTFH-61-83-0-00009, September 1985. The HBS is used with timber poles which are divided lengthwise into several segments. The HBS includes a circular lower connection or slip base, an upper connection or hinge mechanism, and structural support cables. The slip base and hinge mechanism are designed to activate upon impact and are intended to reduce the inertial effects of the pole on the errant vehicle while minimizing the adverse effects on utility service. The circular slip base is also designed to withstand the overturning moments imposed by in-service wind loads and at the same time slip when subjected to the forces of a collision.
The upper hinge mechanism of the HBS is sized so as to adequately support in-service loads while providing for hinging during a collision to allow the bottom segment of the pole to rotate up and out of a vehicle's path. The upper connection reduces the effective inertia of the pole and minimizes the effect of any variation in hardware attached to the upper portion of the pole during a collision. Overhead guys serve to stabilize the upper portion of the pole during a collision and to help insure proper behavior of the upper connection. The HBS is adapted for use specifically in connection with timber utility poles and is designed to provide minimum momentum change for an impacting vehicle and to allow activation by a broad range of vehicle sizes and impact speeds while avoiding the danger of the pole falling on top of the vehicle.
The lower connection of the HBS slip base includes the installation of a circular slip base at an elevation of three inches above grade. This low elevation is intended to avoid snagging the underside of an errant vehicle. The shear plane consists of two 5/8 inch thick plates separated by a 26 gauge keeper plate (intended to maintain a 151/2 inch diameter bolt circle) and by 21/2 inch diameter by 1/8 inch washers. The circular base plates are connected to each other by six 1 inch diameter high strength bolts arranged in circular fashion in slots provided in the circular base plates. The bolts are torqued to 200 ft.-lbs. Connection of the wooden utility pole to the slip base is through a steel pipe or mechanical tubing which is nominally 12 inches in diameter and 30 inches long and is welded to the base plates. In addition, the base plates are braced by 5/8 inch thick stiffeners which are welded to both the base plate and the steel tube.
In the HBS, it is contemplated that the timber pole will fit entirely within the steel pipe or mechanical tube which is welded on the base plate. Moderate trimming of the timber pole can be accomplished without seriously affecting the bending strength of the pole. It is recommended that slip base tube sizes be chosen that will minimize the need for trimming. Any gaps between the tube and the timber pole can be filled with materials such as POLESET which is a 1200 psi, high-density, non-shrinking polyurethane foam available from Utility Structural Systems of Houston, Tex. for use in back filling and protecting poles in the ground.
The upper hinge connection of the HBS system consists of two four-part pole bands installed above and below a saw cut through the pole. The pole bands are secured to the pole by means of bolts. Steel straps are provided to connect the four-part pole bands above and below the saw cut through the pole. One inch diameter bolts pass entirely through the timber pole, both above and below the saw cut, in such a manner as to pass through the pole bands and the steel straps. A vertical slot located below the bolt hole in the bottom of the steel strap is separated from the bolt hole by a small margin of steel. This provides initial bending resistance. However, once the margin is punched out, as by an impacting vehicle, the resistance is greatly lessened and resistance is thereafter offered by friction between the straps and bolts, and by bending of the straps. In addition, the force required to punch out the margin can vary greatly with only small deviations in machining accuracy. Once significant rotation has occurred, the bolts bear on the end of the slot, thereby providing the required ultimate bending strength.
Steel support cables are placed immediately above the upper connection and also near the cross-arm of the utility pole. The lower structural cable serves as a pivot point for the lower pole segment when the pole is struck by an errant vehicle. This cable could be eliminated on poles where a telephone cable is present. The upper steel cable serves to stabilize the upper pole segment and minimize damage to the electrical conductors.
The Hawkins Breakaway System offers many advantages to both utility companies and occupants of impacting vehicles. In almost all cases of vehicle impact, the HBS will prevent conductor damage. In addition, extreme wind gusts, which often cause non-breakaway poles to fail close to grade bringing down and damaging conductors, will not have the same effect on poles modified by the HBS. Such poles will simply bend at the upper connection, absorbing the energy transmitted by the short duration, intense wind gusts, thus preserving the integrity of the transmission facility.
In addition, pole rot is greatly reduced by the installation of the protective sleeve and grouting material around the timber pole in the area most susceptible to pole rot.
Finally, and most importantly, the HBS saves lives by allowing an impacting vehicle to break through a slip base of a timber utility pole rather than be stopped suddenly by the rigid non-modified pole.